What does the RETAIN statement do in a DATA step?

The RETAIN statement is used in a DATA step to keep the value of a variable across iterations of the DATA step. Normally, when a new iteration of the DATA step begins, the value of a variable is reset to missing unless it has been assigned a new value. The RETAIN statement prevents this automatic resetting by holding onto the last value of the specified variable, allowing it to carry the value forward into subsequent iterations.

This functionality is particularly useful in scenarios like creating cumulative sums, running totals, or maintaining flags that depend on previous values in the dataset. By using RETAIN, you ensure that the variable retains its value throughout the execution of the DATA step, thus allowing for more complex data manipulations and calculations.

Other options do not accurately describe the function of the RETAIN statement. For instance, while it may appear that the RETAIN statement could modify a dataset, it actually does not change the structure or contents of the dataset itself. Instead, it influences how individual variable values are treated during the processing of the DATA step.